Yes, I've decided I should post on my blog more often.
Anyway, I got the replacement modem and it works just fine. The failure of the old one was quite weird. Twice I took it to my mother's house and it worked just fine there.
As for IPv6, I wonder if the discrepancy between the apparent 3% to 4% prevalence of IPv6 and the supposedly much larger fraction of ISP customers who have IPv6 might be due to people's OS and/or web browsers preferring IPv4 to IPv6. I thought about testing this by creating dependencies in my web site's style sheet that went to my home computer, some via IPv4 and some via IPv6, to sample who is capable of what, but I didn't feel like writing an HTTP server, and I'll be damned if I'm going to run a publicly-accessible Apache server.
I also tried ipv6-test.com and test-ipv6.com in every web browser I have, and interestingly found that Firefox failed miserably. After some searching on the web, I found many forum posts from people asking how to disable IPv6, and so I did the opposite to allow it. I have no idea why Firefox was configured to disable IPv6 by default. According to what I read on the internet, it shouldn't have been.